This invention relates to an improved power supply for electroluminescent (EL) lamps and, in particular, to a low cost power supply which can be manufactured in integrated circuit form and can be powered by a direct current (DC) source having a voltage of 1-15 volts.
An EL lamp is essentially a capacitor having a dielectric layer including a phosphor powder which glows in the presence of a strong electric field and a very low current. The dielectric layer is held between two electrodes, one of which is transparent. Because the EL lamp is a capacitor, an alternating current (AC) must be applied to cause the phosphor to glow, otherwise the capacitor charges to the applied voltage and the current through the EL lamp ceases.
For wristwatches and other applications such as pocket pagers, an EL lamp is driven by an inverter which converts the direct current from a small battery into alternating current. The battery voltage, typically one to three volts, limits the voltage which can be applied to a lamp by the inverter. In order for the EL lamp to glow sufficiently, a peak-to-peak voltage in excess of one hundred and twenty volts is necessary. Converting from three volts to one hundred and twenty volts is difficult without a transformer and a transformer is too bulky and expensive for a wristwatch and for many other applications.
To increase the voltage across an EL lamp, the prior art teaches connecting the EL lamp across the AC diagonal of a switching bridge and the output from the inverter across the DC diagonal of the bridge. The bridge electrically reverses the connections between the EL lamp and the inverter, reversing the polarity of the applied voltage. The result is an approximate doubling of the voltage across the EL lamp.
A switching bridge of the prior art typically includes four transistors. U.S. Pat. No. 4,527,096--Kindlmann--discloses a bridge circuit including four field effect transistors (FETs). U.S. Pat. No. 4,899,086--Hirata et al.--and European Patent Application 0 372 181 A1--Kamens--disclose a variation in which an SCR is connected in series with a transistor in each half of the bridge. The EL lamp is connected between the junction of the SCR with the transistor in one half of the bridge and the junction of the SCR with the transistor in the other half of the bridge. The European Patent Application also discloses using the gate-anode parasitic capacitance to trigger the SCR when the series transistor is not conducting.
An inverter for EL lamps is typically what is known as a "flyback" inverter in which the energy stored in an inductor is supplied to the EL lamp as a small current at high voltage. FIGS. 1 and 2 are simplified schematic diagrams of flyback inverters in which inverter 10 is a "pumping" inverter and inverter 20 is a "ringing" inverter. The location of diode 16, in part, determines the type of inverter.
Inverter 10 supplies a series of high frequency pulses to lamp 12. A pulse is produced each time transistor 14 turns off, permitting the junction of transistor 14 and series inductor 15 to rise in voltage. Since transistor 14 was conducting, the current through inductor 15 established a field proportional to the current and the inductance of inductor 15. When transistor 14 shuts off, the field collapses at a rate determined by the turn-off characteristics of transistor 14 and the voltage across inductor 15 is proportional to .delta.i/.delta.t. Thus, a low voltage/high current is converted into a high voltage/low current. Each time transistor 14 turns off, the same amount of current is applied to lamp 12 and the voltage on the lamp is pumped by a series of current pulses from the inverter. Diode 16 prevents lamp 12 from discharging through transistor 14.
Inverter 20 operates differently. When transistor 14 shuts off, the energy stored in the field of inductor 15 is applied to lamp 12 until the current through inductor 15 is minimal and the voltage across EL lamp 12 is at a maximum. The current then flows back through inductor 15, increasing the energy stored in the inductor. If there were no losses in the circuit, the process would continue indefinitely, at a frequency determined by the resonant frequency of the inductor and the capacitance of the EL lamp, a phenomenon known as "ringing." Since an AC waveform is being generated, the collector of transistor 14 can go negative relative to the base, drawing leakage current from the battery through the base-collector junction, damping the oscillations. Diode 16 prevents this leakage current. U.S. Pat. No. 4,449,075--D'Onofrio et al.--discloses a ringing inverter.
The frequency of the alternating current through an EL lamp affects the life of the EL lamp, with frequencies below 1,000 hz. being preferred. Too low of a frequency causes a noticeable flicker. Thus, a frequency of 100-1,000 hz. is preferred. Since inductor 15 and lamp 12 form a series resonant circuit, a large inductor is required for resonance at low frequencies. To overcome this problem, a high frequency (10-100 khz.) pulse train is combined with a low frequency pulse train and applied to terminal 19. By using bursts of high frequency pulses, inductor 15 can be made significantly smaller. If a switching bridge is used, the low frequency pulses are applied to the control electrodes of the devices in the bridge.
Stability is a problem in power supplies having a switching bridge, particularly bridges having SCRs. The high frequency, high voltage pulses are coupled to a control electrode of a semiconductor device by the parasitic capacitance inherent in the device, causing erratic triggering. Occasionally, an EL lamp is discharged when it is supposed to be charged and vice-versa. Stability becomes more of a problem as the input voltage to the bridge is increased.
Another problem with power supplies of the prior art is a DC component in the high voltage on the EL lamp. A DC component can cause cathodic reaction in the EL lamp as components of the dielectric layer migrate to one or the other electrode. In FIGS. 1 and 2, the battery is always connected to the EL lamp through inductor 15, causing a DC component equal to the voltage of the DC source voltage. Interposing a switching bridge between the EL lamp and the inverter eliminates the DC component but introduces the stability problem described above.
Another problem with power supplies of the prior art is the forward voltage drop across semiconductor junctions. Each PN junction in a silicon semiconductor device has a forward voltage drop of approximately 0.6 volt. The voltage drop across a saturated transistor can be as low as 0.2 volts. If the inverter of FIG. 1 were used in a watch having a 1.5 volt battery, the maximum voltage across the inductor when transistor 14 is conducting is 1.3 volts. In FIG. 2, the maximum voltage across the inductor is 0.7 volts when transistor 14 is conducting. This limits the high voltage that can be produced. The only way to increase the output voltage is to increase the current through the inductor, putting a severe load on the battery in the watch.
On the high voltage side of the circuit, i.e. the connection to the EL lamp, the problem is not as severe but the forward voltage drop limits the power applied to the EL lamp, particularly if a switching bridge is added. A bridge typically adds a minimum of four junctions in series with the EL lamp.
In view of the foregoing, it is therefore an object of the invention to provide a power supply for operating EL lamps from a DC source having a voltage of 1-15 volts.
Another object of the invention is to provide a power supply which can generate a high voltage across an EL lamp without using a switching bridge.
A further object of the invention is to provide a stable high voltage power supply for EL lamps.
Another object of the invention is to eliminate a DC component through an EL lamp without using a switching bridge in the power supply.
A further object of the invention is to provide a more efficient power supply for an EL lamp.